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SemaphoreFullException Class

The exception that is thrown when the Semaphore.Release method is called on a semaphore whose count is already at the maximum.

Namespace:  System.Threading
Assembly:  System (in System.dll)

public class SemaphoreFullException : SystemException

The count on a semaphore is decremented each time a thread enters the semaphore, and incremented when a thread releases the semaphore. When the count is zero, subsequent requests block until other threads release the semaphore. When all threads have released the semaphore, the count is at the maximum value specified when the semaphore was created. If a programming error causes a thread to call the Semaphore.Release method at this point, a SemaphoreFullException is thrown.


The Semaphore class does not enforce thread identity on calls to the WaitHandle.WaitOne and Semaphore.Release methods. It is not necessary for the same thread that called WaitOne to call Release.

SemaphoreFullException does not necessarily indicate a problem with the code where the exception occurred. Consider the following scenario: Thread A and thread B enter a semaphore that has a maximum count of two. A programming error in thread B causes it to call Release twice, so that the count on the semaphore is full. As a result, when thread A eventually calls Release, a SemaphoreFullException is thrown.

For a list of initial property values for an instance of the SemaphoreFullException class, see the SemaphoreFullException() constructor.

The following code example shows how a programming error in one thread can lead to a SemaphoreFullException in another thread: Two threads enter a semaphore. The second thread releases the semaphore twice, while the first thread is still executing its task. When the first thread finishes and releases the semaphore, the semaphore count is already full and an exception is thrown.

using System;
using System.Threading;

public class Example
    // A semaphore that can satisfy at most two concurrent 
    // requests. 
    private static Semaphore _pool = new Semaphore(2, 2);

    public static void Main()
        // Create and start two threads, A and B.  
        Thread tA = new Thread(new ThreadStart(ThreadA));

        Thread tB = new Thread(new ThreadStart(ThreadB));

    private static void ThreadA()
        // Thread A enters the semaphore and simulates a task 
        // that lasts a second. 
        Console.WriteLine("Thread A entered the semaphore.");


            Console.WriteLine("Thread A released the semaphore.");
        catch(Exception ex)
            Console.WriteLine("Thread A: {0}", ex.Message);

    private static void ThreadB()
        // Thread B simulates a task that lasts half a second, 
        // then enters the semaphore. 

        Console.WriteLine("Thread B entered the semaphore.");

        // Due to a programming error, Thread B releases the 
        // semaphore twice. To fix the program, delete one line.
        Console.WriteLine("Thread B exits successfully.");
/* This code example produces the following output:

Thread A entered the semaphore.
Thread B entered the semaphore.
Thread B exits successfully.
Thread A: Adding the given count to the semaphore would cause it to exceed its maximum count.


Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0